GB2144498A - Roller clutches - Google Patents

Abstract

In a roller clutch comprising an outer race 11 having therein a plurality of circumferentially spaced cam forms 12, an inner race 14, a cylindrical roller 15 interposed between the outer surface of the inner race and each cam form respectively of the outer race, a spring abutment 13 disposed adjacent each of said cam forms, and a spring 19 acting between each roller and the respective spring abutment, the clutch is characterized in that each spring 19 is a helically wound spring and acts against is respective roller 15 through the intermediary of a synthetic resin slipper 21, each slipper 21 having extending therefrom an arcuate synthetic resin spring guide 22 which extends through its respective spring 19 and is slidably received in a bearing bush 17 in the respective spring abutment 13, each slipper/spring guide assembly 21, 22 being guided for circumferential movement relative to the outer race 11 by the bearing bush 17 at one end, and by engagement of the slipper 21 with the outer race 11 at the opposite end. The arrangement thereby minimises the clutch hysteresis. <IMAGE>

Description

SPECIFICATION Roller clutches This invention relates to roller clutches, particularly but not exclusively centrifugual roller clutches, for use in conjunction with internal combustion engines as overrun clutches.

Overrun roller clutches are well known and include a plurality of parallel rollers received between respective cam forms in an outer race and a generally cylindrical surface of an inner race which is mounted concentrically within the outer race. In operation if an attempt is made to induce relative rotation in a first direction between the inner and outer races the rollers jam between their cam forms of the outer race and the cylindrical surface of the inner race to lock the inner race to the outer race. A light spring is associated with each roller and urges its respective roller to the jamming position. If an attempt is made to induce relative rotation of the races in the opposite direction the rollers are moved back slightly against the action of their springs and permit relative rotation between the races.In a starter motor arrangement, the inner race is normally connected to the starter motor pinion and the clutch overruns, that is to say slips, when the engine drives the pinion at a speed exceeding the speed of the outer race. Thus the engine does not drive the starter motor.

During relative rotation of the races the rollers are in light rubbing, or rolling contact with the cylindrical surface of the inner race and the cam forms of the outer race.

Centrifugal roller clutches are less well known but operate in a manner similar to that described above, with the added feature that the cam form design is such that while the clutch is slipping, and when the outer race speed exceeds a predetermined value, centrifugal action on the rollers moves the rollers against the action of their respective springs to a point along their cam forms such that the rollers do not contact the inner race. Thus there is no drag from the rollers on the inner race and wear of the rollers and inner race is minimised. As the speed of the outer race reduces the reverse action occurs, the springs overcoming the centrifugal force acting on the rollers and moving the rollers along their cam forms to make contact with the inner race.It will be understood that there is a difference between the rotational speeds of the outer race at which the rollers disengage from the inner race, and re-engage with the inner race.

This difference is usually known as the "hysteresis" of the system and its magnitude is governed primarily by friction in the roller/spring assembly.

It is an object of the present invention to provide a roller clutch wherein the rollers and their respective springs are accurately guided and located in such a manner that where the roller clutch is a centrifugal roller clutch the hysteresis of the clutch is reduced by comparison with known clutches.

A roller clutch according to the invention comprises an outer race having therein a plurality of circumferentially spaced cam forms, an inner race having a cylindrical outer surface, a cylindrical roller interposed between the outer surface of the inner race and each cam form respectively of the outer race, a spring abutment disposed adjacent one circumferential end of each of said cam forms, and a spring acting generally circumferentially between each roller and the respective spring abutment, the roller clutch being characterized in that each spring is a helically wound spring and acts against its respective roller through the intermediary of a synthetic resin slipper, each synthetic resin slipper having extending therefrom an arcuate synthetic resin spring guide which extends through its respective spring and is slidably received in a bearing bush in the respective spring abutment, each slipper/spring guide assembly being guided for circumferentially movement relative to the outer race by the bearing bush at one end, and by engagement of the slipper with the outer race at the opposite end.

Preferably each spring guide bearing bush is formed from a synthetic resin material so chosen in relation to the synthetic resin material of the spring guide that the bearing exhibits low friction.

Desirably the slipper presents a part-cylindrical surface to the roller, the radius of curvature of the surface being equal to that of the roller, and the axial length of the surface being slightly less than that of the roller.

Conveniently said spring guide is of circular cross-section over part of its length commencing at the slipper, and of non-circular crosssection over that part of its length which passes through the bearing bush in use, said region of non-circular cross-section being equal in its dimension radially of the clutch to the diameter of the region of circular crosssection, but being narrower in its transverse dimension, the bearing bush having a correspondingly shaped aperture therethrough for receiving said non-circular portion of the guide.

Preferably adjacent its free end the guide is formed with latch means which co-operate with the bearing bush to limit circumferential movement of the guide and slipper relative to the bearing bush under the action of the roller spring.

Preferably there is a small clearance between each roller and the end of the spring guide of the next adjacent roller.

Preferably each slipper is integral with its respective spring guide.

Conveniently each of said spring guides is formed from a glass fibre and graphite filled nylon.

Desirably each bearing bush is formed from a PTFE filled acetal material.

Preferably the movement of each spring guide relative to its respective bush in the spring compression direction is limited such that the maximum compression of the spring which can be achieved is insufficient fully to compress the spring.

Conveniently said limit is defined by the engagement of a shoulder, formed on the guide at the point of change of cross-section, with said bush.

One example of the invention is illustrated in the accompanying drawings, wherein: Figure 1 is a diagrammatic side elevational view of part of a centrifugal roller clutch; Figure 2 is a perspective sketch of part of the clutch of Fig. 1; Figure 3 is an exploded perspective view of a spring guide assembly of the clutch of Fig.

1; Figures 4, 5 and 6 are sectional views on the lines 4-4, 5-5, and 6-6 in Fig. 3; and Figure 7 is a view in the direction of arrow A in Fig. 3.

Referring to the drawings, the centrifugal roller clutch includes a forged or machined steel outer race 11 of generally cylindrical form incorporating a driving boss (not shown).

The outer race 11 is hollow, and its inner surface includes a plurality of circumferentially spaced cam forms 1 2. Associated with each cam form 12 and disposed adjacent one circumferential and thereof (the counterclockwise end in Fig. 1) is a spring abutment 13. The spring abutments 1 3 are integral, radially extending, protrusions of the outer race 11.

Disposed concentrically within the outer race 11 is an inner race 14 having a cylindrical outer surface. The inner race 14 is provided with means (not shown) for coupling the race to a driven member. Disposed between each of the cam forms 1 2 and the cylindrical outer surface of the race 14 is a cylindrical steel roller 1 5, the axes of the rollers 1 5 being parallel to the common axis of the races 12, 14. Each of the cam forms 1 2 is inclined with respect to the cylindrical surface of the inner race 14 such that its end remote from the respective spring abutment 1 3 is closer to the cylindrical surface of the race 14 than the end of the cam form adjacent its respective spring abutment 1 3.

Each roller 1 5 can be moved (in a clockwise direction in Fig. 1) to a position in which it jams between its respective cam form 1 2 and the cylindrical surface of the inner race 14 thus locking the inner race 14 to the outer race 11 so that the two rotate as one. In the example shown in Fig. 1, such jamming will occur if an attempt is made to induce clockwise rotation of the race 14 within the race 11, or correspondingly counterclockwise rotation of the race 11 about the race 14.

However, if torque is applied to induce relative rotation of the races 11, 14 in the opposite direction, the jamming action of the rollers will not occur and relative rotation between the races will be permitted.

As is best seen in Fig. 2, each of the spring abutments 1 3 is bifurcated along its radial centreline by means of a circumferentially extending groove 1 6 centered on the axis of the outer race and machined in the inner surface of the outer race 11. The radial depth of the groove is such that it bifurcates the abutments 1 3 and extends into the inner race 11 in that region between the abutments 1 3 and the commencement of their respective cam forms 12, but does not extend into the cam forms 12. Located as a push-fit within the portion f the groove 1 6 in each abutment 1 3 is a moulded synethetic resin bearing bush 17.Desirably each bearing 17 is formed from polytetrafluoroethylene (PTFE) filled acetal plastics material conveniently that available from Dupont under their trade name DELRIN 500AF. Each bush 1 7 includes a cylindrical portion received within the groove 1 6 and at one end an enlarged flange 1 8 which bears against the face of the respective abutment 1 3 presented to the respective roller 1 5.

Acting between the flange 18 of each bush 1 7 and the respective roller 1 5 is a helically wound compression spring 19, the end of each spring 1 9 remote from its respective bush 1 7 abutting a moulded synthetic resin slipper 21 which is in sliding contact with the respective roller 1 5. The face of each slipper 21 which engages a respective roller 15 is of part-cylindrical form having a radius of curvature equal to that of the respective roller 1 5.

The axial length of each slipper is however slightly shorter than the axial length of its respective roller such that each roller protrudes axially at both ends beyond the slipper 21. Each slipper 21 is part of an integral slipper/spring element, the other part of which is defined by an arcuate spring guide 22.

Each spring guide 22 extends from its respective slipper 21 circumferentially of the clutch, and adjacent its opposite end passes through the respective bearing bush 1 7.

Throughout its length the spring guide 22 extends within the spring 19, and over approximately half of its arcuate length, commencing at the slipper 21, each spring guide 22 is of circular cross-section, the diameter of the spring guide being slightly less than the internal diameter of the spring 1 9 so as to support the spring without the convolutions of the spring binding on the surface of the guide 22. Throughout the remainder of its arcuate length each guide 22 is of non-circular crosssection. Fig. 5 shows the cross-sectional shape of the non-circular region and it can be seen that the cross-sectional shape is that which will be achieved by cutting away equal portions symmetrically on opposite sides of a diameter of the circular section. Thus the noncircular region has flat, parallel sides and arcuate top and bottom edges.The non-circular region is so arranged that the flat sides extend radially of the clutch and thus the radial dimension of the non-circular region is equal to the diameter of the circular region. It follows therefore that the support provided by the guide 22 for the spring 1 9 in a direction radially of the clutch is the same throughout the arcuate length of the guide 22. As is best seen in Fig. 7, the aperture extending through the bush 1 7 corresponds in shape to the noncircular cross-section of the guide 22 so that the guide 22 and slipper 21 are guided by the bush 1 7 for sliding movement while rotational movement about the axis of the bush 1 7 is resisted.Moreover movement of the guide 22 relative to the bush 1 7 against the action of the spring 1 9 is limited by engagement of the shoulders defined where the cross-section of the guide changes, with the bush 1 7. The shoulders are so positioned on the guide 22 in relation to the structure of the spring 1 9 that the maximum movement of the guide is insufficient to compress the spring fully.

It can be seen that each slipper 21 is formed with an integral projection 23 which extends towards the outer race 11, and which engages the inner surface of the outer race to provide support for the elements 21, 22 at its end remote from the bush 1 7. The projection 23 is disposed at the midpoint of the slipper 21 (considered in the axial direction of the clutch) and presents a relatively small surface area to the race 11 thus minimising friction.

Although the groove 1 6 does not break the surface of the operative regions of the cam forms 1 2 in this example, it may in certain applications do so, and the projection 23 will then be guided in the groove 16. It is however preferable to ensure that the slipper follows the cam form thus ensuring that the relationship of the slipper to the roller is not disturbed. This is accomplished by the projection 23 riding on the cam form in the example illustrated.

The element 21, 22 is conveniently moulded in a glass fibre and graphite filled nylon material and so is of low weight, and exhibits extremely low friction both at the bearing interface of the projection 23 and the race 11, and at the bearing interface of the guide 22 and bush 17.

Adjacent the free end of the guide 22, each of the flat surfaces formed with a small projection 24 which act as latch means preventing disengagement of the bush 1 7 from the noncircular portion of the guide 22. During assembly of the clutch a sub-assembly is formed by first threading the guide 22 of a slipper/ guide assembly through an appropriate spring 19, and then pressing the free end of the guide 22 through the aperture of a bearing bush 1 7. Sufficient distortion can occur to permit the projections 24 to pass through the aperture of the bush 1 7. However, the load imposed by the spring 1 9 is insufficient to cause the necessary distortion, and thus the projections 24 retain the bush 1 7 on the guide 22 in slight compression between the bush 18 and the slipper 21.

The positioning of the projections 24 is such that in use the travel of the guide 22 relative to the race 11 under the action of the spring 1 9 is sufficient to maintain contact of the slippers 21 and rollers 1 5 even when the rollers 1 5 are on their jamming positions. The projections 24 ensure that a secure sub-assembly of collar 17, guide 22/slipper 21 and spring 1 9 can be handled during assembly of the clutch.

A number of modifications of the arrangement disclosed above are possible. We have already mentioned that the groove 1 6 may be sufficiently deep to break into the cam forms 1 2. It is to be understood that additionally the construction may be in effect radially compressed such that the springs 1 9 are in part received within the groove 1 6.

While the abutments 1 3 may be an integral, radial projection of the race 11 as shown, it is to be understood that they could instead be integral, axial extensions of the rear wall of the race 11. Furthermore the abutments 1 3 could be separate from the body of the race 11, being formed as parts of a pressed steel, or moulded plastic roller cage held in position within the race 11 by any convenient means.

It will be recognised that the slipper/guide element 21, 22 serves to guide and support each spring 1 9 preventing distortion under the centrifugal forces to which the spring is subject. In a centrifugal roller clutch which is to operate at relatively low centrifuging speeds, the springs must provide a relatively low pressure on the rollers and must therefore have a low rate. Inevitably, therefore the springs are very light springs and are easily distorted and buckled by the forces, for example the centrifugal forces acting thereon.

The shaping of the spring guides 22 provides good support for the springs throughout their length while minimising friction in the spring/guide assembly in use. Thus the guides are supported at their ends relative to the outer race, and in turn support the springs 1 9 over the whole of the length of the springs. There is however maintained sufficient clearance between the springs and the guides to avoid the springs gripping or binding against the guides. Thus frictional resistance and drag in the spring/guide arrangements is minimised, thus minimising the hysteresis of the roller clutch.A small circumferential clearance which exists between each guide 22 and the next circumferentially adjacent roller ensures that rubbing contact does not occur between the roller and the end of the guide but at the same time ensures that all of the rollers cantact in unison by virtue of one roller pressing on the guide 22 of the next roller should said next roller and its guide/slipper not move when said one roller moves. The reduction in axial length of the slippers in relation to their respective rollers permits some tilting of the slipper/guide elements if the rollers are caused to skew as can occur during the application of high torque loads through the clutch.

It will be recognised that the guide 22 associated with each spring 1 9 can support each spring so that it is out of contact both with the outer and inner races. In the centrifugal clutch there is thus a significant reduction in the friction between the outer and/or inner race and each spring thus improving the hysteresis characteristic of the clutch. The noncentrifugal roller clutch also benefits from the use of the guides 22 since the reduction of friction between the races and the springs permits the use of springs having lower pressure than is the case in non-centrifugal roller clutches not equipped with the gude 22.In the conventional non-centrifugal roller clutch the presence of high friction between each spring and the outer race necessitates spring pressures higher than theoretically necessary in order to ensure that the rollers are always held positively in contact with the races.

Moreover, frequent causes of failure, primarily but not exclusively, in non-centrifugal roller clutches, are wear of the spring and the inner race because of the spring rubbing against the inner race and misalignment of the springs.

For example, it is usual to use springs of rectangular helical form, and it is known for these to twist through 90 , or to become trapped between the rollers and the cam tracks or the rollers and the inner race. Such failure problems are minimised by the use of guides 22 to locate and support the springs.

In both forms of clutch the machining of the grooves 1 6 through the centreline of the spring abutments permits, in conjunction with the use of the guides 22, efficient utilization of the available space between the inner and outer races, and in other words permits the overall dimensions of the clutch for a given torque capability to be minimised.

Claims (11)

1. A roller clutch comprising an outer race having therein a plurality of circumferentially spaced cam forms, an inner race having a cylindrical outer surface, a cylindrical roller interposed between the outer surface of the inner race and each cam form respectively of the outer race, a spring abutment disposed adjacent one circumferential end of each of said cam forms, and a spring acting generally circumferentially between each roller and the respective spring abutment, the roller clutch being characterized in that each spring is a helically wound spring and acts against its respective roller through the intermediary of a synthetic resin slipper, each synthetic resin slipper having extending therefrom an arcuate synthetic resin spring guide which extends through its respective spring and is slidably received in a bearing bush in the respective spring abutment, each slipper/spring guide assembly being guided for circumferential movement relative to the outer race by the bearing bush at one end, and by engagement of the slipper with the outer race at the opposite end.

2. A clutch as claimed in claim 1, wherein each spring guide bearing bush is formed from a synthetic resin material so chosen in relation to the synthetic resin material of the spring guide that the bearing exhibits low friction.

3. A clutch as claimed in claim 1 or claim 2, wherein the slipper presents a part-cylindrical surface to the roller, the radius of curvature of the surface being equal to that of the roller, and the axial length of the surface being slightly less than that of the roller.

4. A clutch as claimed in any one of claims 1 to 3, wherein said spring guide is of circular cross-section over part of its length commencing at the slipper, and of non-circular cross-section over that part of its length which passes through the bearing bush in use, said region of non-circular cross-section being equal in is dimension radially of the clutch to the diameter of the region of circular cross-section, but being narrower in its transverse dimension, the bearing bush having a correspondingly shaped aperture therethrough for receiving said non-circular portion of the guide.

5. A clutch as claimed in any one of claims 1 to 4, wherein adjacent its free end the guide is formed with latch means which co-operate with the bearing bush to limit circumferential movement of the guide and slipper relative to the bearing bush under the action of the roller spring.

6. A clutch as claimed in any one of the preceding claims, wherein there is a small clearance between each roller and the end of the spring guide of the next adjacent roller.

7. A clutch as claimed in any one of claims 1 to 6, wherein each slipper is integral with its respective spring guide.

8. A clutch as claimed in any one of claims 1 to 7, wherein each of said spring guides is formed from a glass fibre and graphite filled nylon.

9. A clutch as claimed in any one of claims 1 to 8, wherein each bearing bush is formed from a PTFE filled acetal material.

10. A clutch as claimed in any preceding claim wherein, the movement of each spring guide relative to its respective bush in the spring compression direction is limited such that the maximum compression of the spring which can be achieved is insufficient fully to compress the spring.

11. A clutch as claimed in claim 10, where appended to claim 4 and wherein said limit is defined by the engagement of a shoulder, formed on the guide at the point of change of cross-section, with said bush.

1 2. A roller clutch substantially as hereinbefore described with reference to the accompanying drawings.